An iron(ii) complex featuring κ3 and labile κ2-bound PNP pincer ligands-striking differences between CH2 and NH spacers

Article abstract of DOI:10.1039/c4dt01933d

Treatment of anhydrous FeCl₂ with 2 equiv. of the pincer ligand PNP-Ph afforded the diamagnetic cationic octahedral complex [Fe(κ³-P,N,P-PNP)(κ²-P,N-PNP)Cl]⁺ featuring a κ²-P,N-bound PNP ligand. Preliminary reactivity studies revealed that the κ²-P,N-bound PNP ligand is labile reacting with CO to afford trans-[Fe(PNP-Ph)(CO)₂Cl]⁺. This journal is

Full text of DOI:10.1039/c4dt01933d

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3
An iron(II) complex featuring κ and labile
2
κ -bound PNP pincer ligands – striking
Cite this: Dalton Trans., 2014, 43,
14517
differences between CH and NH spacers†
2
Received 26th June 2014,
Accepted 7th August 2014
a
a
b
b
c
Bernhard Bichler, Mathias Glatz, Berthold Stöger, Kurt Mereiter, Luis F. Veiros
a
DOI: 10.1039/c4dt01933d
and Karl Kirchner*
www.rsc.org/dalton
Treatment of anhydrous FeCl
2
with 2 equiv. of the pincer ligand
PNP-Ph afforded the diamagnetic cationic octahedral complex
3
2
+
2
[
Fe(κ -P,N,P-PNP)(κ -P,N-PNP)Cl] featuring a κ -P,N-bound PNP
2
ligand. Preliminary reactivity studies revealed that the κ -P,N-
bound PNP ligand is labile reacting with CO to afford trans-
+
[
Fe(PNP-Ph)(CO)
2
Cl] .
Tridentate PNP ligands based on a central pyridine donor with
CH PR , –OPR , or –NHPR substituents in the two ortho
positions are widely utilized in transition metal chemistry (e.g.,
Fe, Ru, Rh, Ir, Pd, Pt). Most of these PNP pincer complexes
feature strongly electron donating and bulky phosphine
donors such as iPr or tBu substituents, while complexes with
less bulky and less electron donating Ph groups are, in com-
parison, less utilized.
Scheme 1 Formation of 16e iron(II) PNP pincer complexes of the type
–
2
2
2
2
2
[Fe(PNP)Cl ].
1
–3
iron PNP complexes, we report here on the synthesis and reac-
tivity of an unusual octahedral Fe(II) complex where two N,N′-
bis(diphenylphosphino)-2,6-diaminopyridine (PNP-Ph) pincer
3
2
ligands are coordinated both in κ (P,N,P)- and κ (P,N)-fashion.
In sharp contrast to the reactions shown in Scheme 1, treat-
As far as iron is concerned, coordinatively unsaturated 16e
ment of anhydrous FeCl
ence of AgBF4 at room temperature for
diamagnetic emerald green cationic octahedral complex [Fe(κ -
2
with PNP-Ph (1) in THF in the pres-
iron(II) complexes of the type [Fe(PNP)Cl
2
] were prepared by
CH
2
4
h
afforded
reacting FeCl with the pyridine-based PNP ligands PNP
2
-
3
CH
2
4
CH
2
5,6
tBu, PNP -iPr, and also PNP -Ph, featuring CH spacers
2
2
+
P,N,P-PNP-Ph)(κ -P,N-PNP-Ph)Cl] (2) in essentially quantitative
between the phosphine donors and the pyridine moiety
+
yield (Scheme 2). In the absence of Ag salt as halide scaven-
(Scheme 1). The bulkiness and donor strengths of these
ger, the respective chloride complex 2Cl was formed. The for-
mation of these complexes is independent of whether 1 or
ligands decrease in the order PNP-tBu > PNP-iPr > PNP-Ph and
apparently seems to have no influence on these reactions. We
have recently prepared similar complexes but with the bulky
PNP ligands N,N′-bis(di-iso-propylphosphino)-2,6-diamino-
pyridine (PNP-iPr) and N,N′-bis(di-tert-butylphosphino)-2,6-
diamino-pyridine (PNP-tBu) where the central pyridine ring
contains NHPR2 (R = iPr, tBu) substituents in the two ortho
2
equiv. of ligand are used.
However, in the first case substantial amounts of unreacted
FeX
2
remained which form the paramagnetic haloferrate coun-
2
−
terion [FeCl ] . The reaction of FeCl with the related ligand
4
2
CH
2
CH
2
PNP -Ph yields the pentacoordinate complex [Fe(PNP -Ph)-
Cl ] revealing striking differences between CH and NH
1
g,7
positions (Scheme 1).
In continuation of our studies on
2
2
a
Institute of Applied Synthetic Chemistry, Vienna University of Technology,
Getreidemarkt 9, A-1060 Vienna, Austria. E-mail: kkirch@mail.tuwien.ac.at
b
Institute of Chemical Technologies and Analytics, Vienna University of Technology,
Getreidemarkt 9, A-1060 Vienna, Austria
c
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa,
Av. Rovisco Pais No. 1, 1049-001 Lisboa, Portugal
†
Electronic supplementary information (ESI) available: Experimental and com-
putational details, spectroscopic characterization. CCDC 1005379 for 2. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c4dt01933d
3
2
+
Scheme 2 Formation of [Fe(κ -P,N,P-PNP)(κ -P,N-PNP)Cl] (2) featur-
ing a κ -P,N bound PNP ligand.
2
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5
,6
spacers in pyridine-based PNP pincer ligands (Scheme 1).
The formation of 2 is very selective and only one isomer was
2
observed where the pyridine moiety of the κ -P,N bound PNP
ligand is trans to the halide ligand. There was no evidence for
the formation of an isomer where the pyridine moiety of the
2
κ -P,N bound PNP ligand is cis to the halide ligand. DFT calcu-
lations for the two possible isomers of 2 in agreement with the
3
2+
3
Scheme 3 Formation of [Fe(κ -P,N,P-PNP)
P bound PNP ligands.
2
]
(3) featuring two κ -P,N,
experiment indicate that the unobserved isomer is less stable
by 6.9 kcal mol−1. That stability difference is essentially due to
steric effects (for details see ESI†).
CH
2
2+
Complex 2 is fully characterized by a combination of has to be noted that an analogous complex [Fe(PNP -Ph)]2
1
31
1
H and P{ H} NMR spectroscopy, and elemental analysis. bearing CH -spacers between the pyridine ring and the PPh
2
2
1
9
While the H NMR spectra were not very informative, the moieties was reported.
3
1
1
8
P{ H} NMR spectrum revealed a characteristic A B pattern.
In a preliminary study we investigated the reactivity of [Fe-
2
3
2
+
The spectrum gives rise to multiplets centered at δ = 108.7 (κ -P,N,P-PNP-Ph)(κ -P,N-PNP-Ph)Cl] (2) towards the addition
A
3
2
(
κ -PNP) and δ
B
= 100.9 (κ -PNP) with a coupling constant JPP of CO. Treatment of 2 with CO at room temperature resulted in
2
of 50 Hz, which can be assigned to the two phosphorus atoms the dissociation of the κ -P,N-bound PNP ligand and formation
3
3
of the κ -PNP-bound ligand, and one phosphorous atom of of the cationic dicarbonyl complex trans-[Fe(κ -P,N,P-PNP-Ph)-
2
+
κ -PNP-bound ligand. In addition, the pendant PR
2
-NH-arm of (CO) Cl] (4) as shown in Scheme 4.
2
2
the κ -bound PNP ligand gives rise to a singlet at 47.0 ppm.
Complex 4 is a thermally robust red solid that is air stable
The solid state structure of 2, determined by single-crystal both in the solid state and in solution for several days. In the
1
3
1
X-ray diffraction is depicted in Fig. 1 with selected bond dis-
C{ H} NMR spectrum the two CO ligands exhibit a single
tances given in the caption. This complex adopts a severely dis- low-intensity triplet resonance 207.2 ppm with a coupling con-
torted octahedral geometry with the P1–Fe1–P2, N1–Fe1–P3, stant JCP of 25.8 Hz, thus, clearly revealing that the two CO
and N4–Fe1–Cl1 angles deviating from the ideal 180° being ligands are trans to one another. Accordingly, 4 gives rise to
only one band 2031 cm− in the IR spectrum for the mutually
1
1
70.02(6), 163.33(3), and 172.36(6)°, respectively.
Complex 2 is stable in THF for several days, but readily trans CO ligands which can be assigned to the asymmetric CO
rearranges in CH CN (within a few minutes) and CH Cl
stretching frequency. The symmetric CO stretching band is IR
within a few hours) to give the known complex [Fe(κ -P,N,P- inactive and thus not observed. For comparison, in the related
3
2
2
3
(
2
+
+
PNP-Ph)
2
]
(3) (Scheme 3). In the presence of halide scaven- complex trans-[Fe(PNP-iPr)(CO) Cl] the CO stretch is shifted
2
gers this reaction proceeds within a few minutes. Complex 3 to lower wave numbers observed at 2015 cm− , clearly showing
1
7
a
7e
was already prepared by an alternative method recently. It that PNP-iPr is a stronger electron donor than PNP-Ph.
In summary we prepared the octahedral Fe(II) complex [Fe-
3
2
+
(
κ -P,N,P-PNP-Ph)(κ -P,N-PNP-Ph)Cl] (2) where the PNP pincer
3
2
ligands are coordinated both in κ -P,N,P- and κ -P,N-fashion.
This complex adopts a strongly distorted octahedral geometry
as established by X-ray crystallography. Obviously related to
these distortions is the reactivity of 2 undergoing, upon halide
dissociation, a facile rearrangement reaction to give the di-
3
+
cationic complex [Fe(κ -P,N,P-PNP-Ph) ] (3) where now both
2
3
PNP-Ph ligands are bound in κ -P,N,P-fashion. Moreover, pre-
liminary reactivity studies revealed that the κ -P,N-bound
2
PNP-Ph ligand is substitutionally labile resulting in dis-
2
sociation of the κ -P,N-bound PNP ligand in the presence of
3
CO to afford the cationic dicarbonyl complex trans-[Fe(κ -P,N,P-
+
PNP-Ph)(CO)
2
Cl] (4) and free PNP-Ph ligand. In order to
3
2
Fig. 1 Structural diagram of [Fe(κ -P,N,P-PNP-Ph)(κ -P,N-PNP-Ph)Cl]
BF
4
·2THF·Et
2
O (2·2THF·Et
2
O) showing only N-bonded H-atoms and
−
40%-ellipsoids (solvent molecules and BF
4
counterion are omitted for
clarity). Selected bond distances and angles (Å, °): Fe1–N1 2.064(2), Fe1–
N4 2.084(2), Fe1–P3 2.1845(7), Fe1–P1 2.2439(7), Fe1–P2 2.2530(7),
Fe1–Cl1 2.3330(7); P1–Fe1–P2 163.33(3), N1–Fe1–P3 170.02(6),
N4–Fe1–Cl1 172.36(6), C34–N6–P4 120.3(3); hydrogen bond N6⋯N1
3
2
+
Scheme 4 Reaction of [Fe(κ -P,N,P-PNP-Ph)(κ -P,N-PNP-Ph)Cl] (2)
2.941(3).
with carbon monoxide.
14518 | Dalton Trans., 2014, 43, 14517–14519
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answer the question whether these reactions are sterically or
electronically driven, investigations are currently underway uti-
lizing PNP ligands with both weakly and strongly electron
donating substituents which however are sterically non-
demanding. These results will be reported in due course.
Financial support by the Austrian Science Fund (FWF) is
gratefully acknowledged (Project no. P24202-N17) and LFV
acknowledges Fundação para a Ciência e Tecnologia, Projecto
Estratégico – PEst-OE/QUI/UI0100/2013. The X-ray center of the
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Schwartsburd,
M.
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5
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Dalton Trans., 2014, 43, 14517–14519 | 14519